Method for decreasing emissions of nitrogen oxides and sulfur oxides when burning fuels which contain nitrogen and sulfur

ABSTRACT

The invention relates to a method for decreasing emissions of nitrogen oxides and sulfur oxides when burning fuel which contains nitrogen and sulfur. According to the invention this is carried out by feeding fuel (4) and oxygen-containing gas (5) into a combustion reactor (1) the temperature of which is preferably 900°-1500° C., and that the combustion gases formed are directed into a suspension reactor (2) the temperature of which is preferably 750-1050° C. and into which a pulverous material which binds sulfur oxides is fed so that the suspension density is 1-200 kg/m 3 , the oxygen concentration in the combustion reactor (1) and the suspension reactor (2) being controlled so that their total air coefficient is about 0.65-2, whereafter the gases are directed into an after-treatment reactor (3), into which oxygen-containing gas (12) is fed in order to adjust the oxygen concentration in the flue gases so that the residual oxygen concentration in the flue gases (13) emerging from the after-treatment reactor is 0.5-16, preferably 1-6% by volume.

The present invention relates to a method for decreasing emissions ofnitrogen oxides and sulfur oxides when burning a fuel which containsnitrogen and sulfur. The method is based on the control of thecombustion process so as to decrease the formation of nitrogen oxidesand/or on the reduction of nitrogen oxides present in the flue gases andon the binding of the sulfur oxides present in the flue gases to apulverous material. The method is especially suitable for the treatmentof gases produced from a solid fuel such as pulverized coal.

The burning of fossil fuels produces sulfur oxides and nitrogen oxides,which are deleterious to the environment; the environmental hazards dueto them became central problems in energy technology in the late 1970s.In Japan, the United States and certain Western European countries,statutory norms have been set regarding the maximum allowed emissions ofsulfur dioxides (SO_(x)) and nitrogen oxides (NO₂), and most likelythere will be corresponding developments in all industrialized countriesin the near future.

In the known solutions to the problem, emissions of nitrogen oxides arelimited primarily by affecting the combustion process so that oxides ofnitrogen are formed at a minimal rate. A method known to be effective isto introduce the combustion air in steps so that the pyrolysis andpreoxidation of the fuel occur in substoichiometric conditions. Therebythe nitrogen bound in the organic part of the fuel is at least in partrendered to the form of a stable N₂ molecule, whereupon its oxidationremains low. In general, the maximum temperature in the combustionchamber can also be limited by introducing the air in steps, and thishas a decreasing effect on the so-called thermal NO_(x). The returningof cold flue gases into the combustion chamber has a similar effect.

It is also known to reduce NO_(x) by means of special catalyst reactors,in which the reduction of nitrogen oxide to molecular nitrogen isaccomplished, usually by means of ammonia (NH₃).

By pyrotechnical means alone it is not possible to comply with thestrictest NO_(x) norms in solid-fuel boilers, but in these cases it isalso necessary to treat in the above-mentioned catalyst reactors atleast a proportion of the flue gases.

The known catalyst reactors for NO_(x) are solid-bed or cell structurescoated with a catalyst material; these structures typically operate attemperatures of 300°-400° C., and the reductant most commonly used isammonia gas (NH₃). In order to accomplish a good mass transfer contact,the gas conduits formed by the catalyst sheets must be oblong and thehydraulic diameter of the conduits must be small. Since a large amountof catalyst surface is needed, the catalyst reactor must be constructedso as to be uncooled. From this it follows that it is not advantageousto raise the operating temperature of catalyst reactors above 400° C.Present-day catalyst reactors are not suitable for fuels containingsulfur. In terms of the NO_(x) reactors, fuels which contain both ashesand sulfur, such as coal, are very problematic.

The disadvantages of catalyst reactors include high investment costs andconsiderable operating costs. Furthermore, practical experience hasshown that during operation the catalyst sheets lose some of theircatalytic effect owing to soiling and poisoning. One important cause ofsuch poisoning is SO₃, which in general sulfates an oxidic catalystmaterial. In the form of sulfate the catalyst material loses its effect.Another central problem encountered in the reduction of nitrogen oxidespresent in sulfur-containing gases by means of ammonia gas in catalystreactors is the corroding and soiling of the air heaters of the steamboilers, caused by the forming ammonium sulfates. In the known reductionreactors for nitrogen oxides it has not been possible to solve theseproblems. The known catalyst reactors have had a further disadvantage inthe compounds, detrimental to the environment, resulting from theunreacted ammonia residues. In some cases wear occurs in the catalystcell systems, and problems of clogging have also been reported. Onesubstantial problem in present-day catalyst reactors is that theregeneration of the catalyst material is difficult or impossible.

By the methods of nitrogen oxide removal it is usually not possible toaffect the emissions of SO_(x), and for this purpose it is in generalnecessary to install separate devices, which for their part are notcapable of substantially removing the oxides of nitrogen. A large numberof different methods have been developed for the removal of SO_(x) fromcombustion gases, and at least the following have attained significance:

(1) direct binding in wet scrubbers of sulfur oxides in the form ofsulfate,

(2) direct binding of sulfur oxides in the form of sulfate by theso-called semi-wet method,

(3) direct binding in the combustion chamber of sulfur oxides in theform of sulfate (or sulfide), and

(4) binding of sulfur oxides in a regeneratable medium.

Method 1 has already been applied in practice in several plants, and itcan perhaps be regarded as a method which has reached the commercialstage. Its disadvantages include various problems of wear and clogging,high costs of operating, and problems due to the effluents produced.

Method 2 in various forms has also been applied in practice. Itsgreatest problems pertain to the control of the humidity conditions inthe apparatus. If the absorption material dries too quickly, theabsorption of SO_(x) remains poor. On the other hand, the condensing ofwater vapor causes availability problems. The fiber filter most commonlyused for the separation of the absorption material is especiallysensitive to water. The semi-wet method requires a very precise controlof the temperature and the moisture, which substantially hampers theapplication of this method to production.

Method 3 is preferably applied in connection with fluidized-bedcombustion, the fluidized material containing calcium. When thecombustion temperature is about 800°-900° C., SO_(x) will combine withcalcium in the form of sulfate. The method is simple and does not causeavailability problems as do methods 1 and 2.

Method 3 has a disadvantage in the narrow temperature range required byits effective application and in its high Ca/S molar ratio (a separationof 30-80% usually presupposes that the fresh Ca/S feed ratio is over 2).

Methods 4 are mainly at the stage of being developed. What they have incommon is that SO_(x) is in general absorbed from combustion gases freedof solids, into a solution or into a solid at a temperature at which theabsorption is effective. By heating the absorption material, an SO_(x)-containing gas is obtained, and at the same time the absorptionmaterial is regenerated for reuse for the absorption of SO_(x). All ofthe known methods of group 4 are characterized by high costs ofinvestment and operation. Since the methods require complicatedapparatus, they usually also involve usability problems. An additionalproblem consists of the further treatment of the SO_(x) -rich gas, inwhich the sulfur is finally bound either as elemental sulfur or assulfuric acid. It is clear that the elemental sulfur or sulfuric acidobtained as a product does not suffice to compensate for the high costsof investment and operation of the method.

It is evident that the apparatuses for the removal of sulfur oxides andnitrogen oxides are expensive when the present-day techniques areapplied, and at the same time the availability of the power plants islowered. A special problem consists of the solid-fuel boilers to whichit is difficult or impossible, both technically and economically, toconnect SO_(x) and NO_(x) removing devices.

The object of the present invention is to provide a method fordecreasing emissions of nitrogen oxides and sulfur oxides in connectionwith the burning of a fuel which contains nitrogen and sulfur, a methodby which the oxides of nitrogen and sulfur can be removed effectivelyfrom the combustion gases in a simple and economical manner.

By the method according to the invention, both reduction of NO_(x) andeffective absorption of SO_(x) are accomplished in one and the samesimple apparatus. By the method according to the invention it is alsopossible to affect the combustion process so that the formation ofNO_(x) is kept at a low level. The method can be applied to both old andnew boilers, regardless of the burning technique otherwise applied inthe boiler.

According to one preferred embodiment of the method according to theinvention, preoxidation of a fuel which contains nitrogen and sulfur iscarried out by feeding fuel and air or some other oxygen-containing gasinto a combustion reactor, the temperature of which is preferably900°-1500° C., so that the air flow is maintained at a level below thestoichiometric level, the air coefficient being about 0.5-0.95. Owing tothe reducing conditions prevailing in the combustion reactor, most ofthe nitrogen present in the fuel is rendered to the form of molecularnitrogen, and so the formation of nitrogen oxides is low. At the sametime the temperature in the combustion reactor can be regulated easilyby adjusting the air coefficient within a range below the stoichiometriclevel. The gases emerging from the combustion reactor are led into asuspension reactor, into which a pulverous material required for thebinding of the sulfur oxides is also fed; this material is preferably amaterial which contains alkali or alkali earth compounds, such ascalcium carbonate, calcium-magnesium carbonate, or a correspondingoxide. In the suspension reactor there is a change to oxidizingconditions, and the temperature is selected so as to be suitable for thebinding of sulfur, i.e. about 750°-1050° C. in the case of calcium-basedabsorption materials. In this case the pulverous absorption material canbe caused to calcinate into the said pulverous material, mostly in theform of a stable sulfate. The adjustment of the temperature can becarried out by means of cooled surfaces placed in the suspensionreactor. From the suspension reactor the gases are directed into anafter-treatment reactor, and their oxygen content is regulated by meansof an air flow directed into the connecting part between the suspensionreactor and the after-treatment reactor. The temperature of the gasesarriving in the after-treatment reactor is preferably above 800° C., andin this case the final oxidation is achieved in the after-treatmentreactor.

According to another preferred embodiment of the invention,superstoichiometric combustion is used in the combustion reactor, and inthis case a reductant is added to the suspension reactor in order toreduce the nitrogen oxides. The reduction reaction can be enhanced byadding a catalyst to the suspension reactor, the catalyst preferablybeing a material which contains compounds of iron and/or copper,preferably oxide, silicate and/or hydroxide.

According to the invention, it is also possible to reduce the oxides ofnitrogen in the suspension reactor by exploiting the coke particles andcombustible gases present in the combustion gases.

The invention is described below in greater detail with reference to theaccompanying drawing, which depicts diagrammatically an apparatussuitable for carrying out the method according to the present invention.

The main operations of the method according to the invention take placein the combustion chamber 1, the suspension reactor 2 (i.e., anentrained fluidized bed-type reactor) and the after-treatment reactor 3.

The sulfur- and nitrogen-containing material 4 to be burned is fed intothe combustion chamber 1, into which air 5 is also introduced. The rateof the air flow 5 is proportioned to the fuel flow 4 in such a way thatthe conditions in the combustion chamber 1 will be reducing. Thetemperature of the combustion chamber can, when necessary, be set tocontrol the air flow 5, whereby at the same time the problems due to themelting of the ashes, for example, can be avoided. Under the effect ofthe reducing conditions prevailing in the combustion chamber 1, theconcentration of nitrogen oxide in the gases arriving in the reactor 2will be low.

The gases emerging from the combustion chamber 1 are directed throughthe nozzle 6 into the reactor part 2, into which the pulverous material7 required by the binding of sulfur is also directed. It is alsopossible to feed into the reactor 2 a gaseous or solid reductant 8 and apulverous catalyst 9, in order to reduce the nitrogen oxides produced inthe combustion chamber.

In order to regulate the oxygen concentration required by the binding ofsulfur, air 10 is fed into the reactor 2. If the sulfur is bound insulfate form, the suitable molar proportion of available oxygen is0.1-1.0% and the suitable reactor 2 temperature is 800°-1050° C. In thecombustion chamber 1 the nitrogen present in the fuel 4 has in the mainbeen rendered to the form of molecular nitrogen, and so the formation ofnitrogen oxides under the above-mentioned conditions required by theformation of sulfates is insignificant. If the binding of sulfur in thereactor 2 is based on the formation of sulfides, it also serveseffectively to reduce nitrogen oxides. The reduction can be promoted byusing catalysts 9. Because of the simultaneous binding of sulfur oxidesand the relatively high temperature, the formation of SO₃ is practicallynil, and so the poisoning of the catalyst is avoided. Owing to thepulverous form of the catalyst material it is possible to obtain a largecontact surface, and the fluidized or pneumatically carried catalystparticles to be recycled are automatically cleaned of solid impurities.

The oxygen concentration in the gases emerging from the reactor 2 isregulated by adjusting the air flow 12 entering the mixing part 11between the reactor 2 and the after-treatment reactor 3. The temperatureof the gases emerging from the reactor 2 is over 800° C., and so a finaloxidation is achieved in the after-treatment reactor, in which case anyexcess amounts of reductant compounds emerging from the reactor 2 aredestroyed by oxidation. The after-treatment reactor 3 can be, forexample, a centrifugal separator, in which case the pneumaticallycarried particles can at the same time be separated from the emerginggases 13 and be returned to the reactor 2 through unit 14.

The powder which has been uxsed to bind sulfur and the catalyst used canbe removed from the reactor 2 through the unit 15.

The optimal reaction conditions depend on the fuel used in each case.When, for example, pulverized coal combustion is used, the conditions inthe combustion reactor 1 are preferably selected as follows:

    ______________________________________                                        Combustion reactor (1)                                                        temperature max         1400° C.                                       air coefficient         0.70                                                  Suspension reactor (2)                                                        temperature             850° C.                                        air coefficient in      0.70-                                                 out                     1.15                                                  After-treatment reactor (3)                                                   temperature             850-1000° C.                                   air coefficient         1.15                                                  ______________________________________                                    

The invention is described below in greater detail with the aid ofexamples.

Example 1

Pulverized coal combustion:

A substoichiometric combustion is carried out in the combustion reactor,the air coefficient being 0.65. The molar proportions of the reducingcompounds present in the gas, divided by the molar proportions of thegaseous compounds are, upon emerging from the combustion reactor

C(s); 0.12

CO; 0.08

H₂ ; 0.11

CH₄ ; 0.01

In addition, the gases contain small amounts of other reducingcompounds, such as aliphatic hydrocarbon and cyano compounds and otherorganic nitrogen compounds, as well as intermediate products of thereactions occurring in the process and aromatic carbon compounds. Thenitrogen oxides present in the gases are primarily nitrogen monoxide(NO), and their molar proportion in the gas compounds emerging from thecombustion reactor is 166 ppm. The temperature of the gas in thesuspension reactor is nearly constant and adjusted by means of coolingto the value 850° C. In the suspension reactor the further oxidation ofthe compounds pdresent in the gas emerging from the combustion reactoris carried out by directing an air flow into the lower part of thesuspension reactor, the total air coefficient thereupon increasing to0.95. In the gases emerging from the suspension reactor the molar flowsof the reducing compounds, divided by the molar flow of the gaseouscompounds, are

C(s); 0.008

CO; 0.015

H₂ ; 0.020

CH₄ ; 0.001

Further reduction of the oxides of nitrogen takes place in thesuspension reactor under the influence of solid carbon and reducing gascompounds so that the molar proportion of NO_(x) in the gases emergingfrom the suspension reactor will have decreased to 45 ppm.

In order to bind the oxides of sulfur, lime in pulverous form is fedinto the suspension reactor. The concentration of sulfur in the coal tobe burned is 0.4 mol/kg, and in order to bind the sulfur, lime is fedinto the suspension reactor so that the ratio of lime to fuel is 0.75mol/kg. The oxides of sulfur are bound in the suspension reactor mainlyin the form of calcium sulfate and to a small extent as calcium sulfite,whereupon the molar proportion of SO₂ in the gases emerging from thesuspension reactor will be 130 ppm.

The final oxidation of the reducing compounds is carried out in theafter-treatment reactor, whereby the total air coefficient increases to1.2. In the final oxidation the molar proportion of NO_(x) in theemerging gases will at the same time increase to 80 ppm.

Example 2

Combustion of coal (sulfur concentration 0.4 mol/kg):

Substoichiometric combustion is carried out in the combustion reactor,the air coefficient being 0.9. After the combustion the concentrationsof the most important reducing compounds (solid carbon, carbon monoxide,hydrogen, methane) in the gases (molar proportion of the compound to thegaseous compounds) are as follows:

C(s); 0.002

CO; 0.025

H₂ ; 0.030

CH₄ ; 0.001

The post-combustion-reactor temperature is 1300° C. and the total molarproportion of nitrogen oxides in the gas compounds is 300 ppm.

Air is added to the suspension reactor so that the totalpost-suspension-reactor air coefficient is 1.1. Ammonia is fed into thesuspension reactor so that the ratio of ammonia to fuel is 135 mmol/kg.The temperature in the suspension reactor is adjusted to 930° C. bymeans of cooling. In the suspension reactor the nitrogen oxides arereduced under the influence of ammonia so that the NO_(x) concentrationin the emerging gas flow will be 85 ppm.

Lime is also fed into the suspension reactor so that the ratio of limeto fuel is 0.83 mol/kg. The lime is fed in the form of a powder theparticle size of which is mainly within the range 0.05-1 mm. The densityof solids in the suspension reactor is adjusted to a value within therange 5-100 kg/m³ by removing the coarsest fraction of the solidsthrough a withdrawal unit.

In the after-treatment reactor the gases coming from the suspensionreactor are oxidized by feeding into them air so that the total aircoefficient will be 2.3.

Example 3

Burning of coal:

90% of the fuel flow is introduced into the combustion reactor, and thisproportion is oxidized in the combustion reactor, the air coefficientbeing 1.0. In the gases emerging from the combustion reactor, the molarproportion of the reducing compounds to the gaseous compounds is

C(s); 0.0018

CO; 0.0130

H₂ ; 0.0160

CH₄ ; 0.0001

In the after-treatment reactor the reducing compounds are oxidized sothat the total air coefficient will be 1.15, whereupon the concentrationof nitrogen oxides will be 90 ppm.

Example 4

Suspension burning of coal (sulfur concentration 0.4 mol/kg):

Superstoichiometric combustion is carried out in the combustion reactor,the air coefficient being 1.05, whereafter the temperature is 1150° C.and the NO_(x) concentration in the gaseous compounds is 370 ppm. Thegases are directed from the combustion reactor to the suspensionreactor.

In order to bind the oxides of sulfur, lime in pulverous form is fedinto the suspension reactor so that the ratio of lime to fuel is 0.9mol/kg. In addition, in order to reduce the oxides of nitrogen, ammoniaand a pulverous material which contains oxides of copper and/or iron arefed into the suspension reactor. The ratio of ammonia to fuel is 165mmol/kg and the mass ratio of the pulverous material which containscopper and iron oxides to fuel is 0.01-0.05. The mean particle size ofthe powder used as a catalyst is typically 0.05-1.0 mm. The density ofthe suspension in the suspension reactor is regulated, when necessary,by withdrawing the coarsest material through a unit located in the lowerpart of the reactor.

In the suspension reactor, oxides of nitrogen are reduced so that themolar proportion of NO_(x) in the gaseous compounds emerging from thesuspension reactor is 80 ppm. The oxides of sulfur are mostly bound inthe pulverous, lime-containing material so that the molar proportion ofSO₂ in the gaseous compounds in the gas emerging from the suspensionreactor is 97 ppm.

The oxidation of the organic compounds and carbon monoxide, present inlow concentrations, is carried out to completion in the after-treatmentreactor, whereupon the total air coefficient is 1.15.

We claim:
 1. A method for decreasing emissions of nitrogen oxides andsulfur oxides in the combustion of a fossil fuel which contains nitrogenand sulfur, the method being based on the regulation of the combustionin order to decrease the formation of nitrogen oxides and/or on thereduction of the nitrogen oxides present in the flue gases, and on thebinding, into a pulverous material, of the sulfur oxides present in theflue gases, characterized in that the fuel and a stoichiometric orhigher than stoichiometric amount of oxygen-containing gas are fed intoa combustion reactor so as to provide an air coefficent which is about1-2 and to burn the fuel, the temperature of the combustion reactorbeing 900°-1500° C., and so that the resulting combustion gases containan oxygen content and are directed into a suspension reactor thetemperature of which is 750°-1050° C. and into which a pulverousmaterial which binds sulfur oxides is fed to provide a combustiongases-pulverous material suspension having a density of 1-200 kg/m³, theoxygen concentration in the combustion reactor and the suspensionreactor being adjusted so that the total air coefficient in saidcombustion reactor and in said suspension reactor is about 1-2, thegases then being directed into an after-treatment reactor, into which anoxygen-containing gas is fed for adjusting the oxygen content in theresulting flue gases so that the residual oxygen content in the fluegases emerging from the after-treatment reactor is 0.5-16% by volume. 2.A method according to claim 1, including feeding a reductant into thesuspension reactor in order to reduce the nitrogen oxides.
 3. A methodas in claim 2 wherein the reductant is gaseous ammonia.
 4. A method asin claim 2 wherein the alkaline material is selected from the groupconsisting of calcium carbonate, calcium-magnesium carbonate andcorresponding oxides.
 5. A method according to claim 1, includingfeeding into the suspension reactor a pulverous reduction catalyst.
 6. Amethod as in claim 3 wherein the reduction catalyst is a material whichcontains compounds of iron or copper.
 7. A method as in claim 6 whereinthe compounds are selected from the group consisting of oxides,silicates and hydroxides.
 8. A method according to claim 1, wherein thepulverous material used for the binding of sulfur oxides is alkaline. 9.A method as in claim 4 wherein the pulverous material is alkaline.
 10. Amethod according to claim 1 wherein the pulverous material is calciumcarbonate, calcium-magnesium carbonate, or a corresponding oxide.
 11. Amethod according to claim 1 wherein solid particles are separated fromthe gases in the after-treatment reactor and at least a portion of theparticles thus obtained are returned to the suspension reactor.
 12. Amethod according to claim 1 wherein the combustion gases entering thesuspension reactor contain coke particles and combustible gases, andwherein the coke particles and the combustion gases are used for thereduction of nitrogen oxides.
 13. A method as in claim 1 wherein theresidual oxygen content in the flue gases emerging from theafter-treatment reactor is adjusted to 1-6% by volume.
 14. A method fordecreasing emissions of nitrogen oxides and sulfur oxides during burningof a fossil fuel which contains nitrogen and sulfur, the methodcomprising: burning the fuel in a combustion reactor with astoichiometric or higher than stoichiometric amount of oxygen-containinggas at air coefficient of about 1 to 2 and at a temperature of 900° C.to 1500° C. thereby forming combustion gases which contain oxygen;directing the combustion gases from the combustion reactor into asuspension reactor and feeding into the suspension reactor anoxygen-containing gas and a pulverous alkaline binder thereby forming asuspension thereof in which the binder reacts with sulfur oxides in thecombustion gases to form a suspended pulverous solid reaction productcontaining sulfur, the temperature in the suspension reactor being 750°C. to 1050° C. and the suspension having a density of 1-200 kg/cubicmeter; adjusting the oxygen concentration in the combustion reactor andin the suspension reactor so that the total air coefficient thereof isabout 1 to 2; directing the suspension and the gases and anoxygen-containing gas into an after-treatment reactor wherein anyorganic compounds and carbon monoxide formed during burning of the fuelare oxidized to completion; adjusting the oxygen content of the gases inthe after-treatment reactor to 0.5-16%; separating the solids from ofgases in the after-treatment reactor; and discharging the gases as fluegases.